High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) is a technology that is capable of separating gas-phase ions at atmospheric pressure. In FAIMS, the ions are introduced into an analyzer region across which a radio frequency (rf) waveform, the magnitude of which is referred to as dispersion voltage (DV), is applied such that the ions are alternately subjected to high and low electric fields. The waveform is asymmetric; for example, the high field may be applied for one time unit followed by an opposite-polarity low field of half of the high field component applied for twice as long. The field-dependent change in the mobility of the ions causes the ions to drift towards the walls of the analyzer region. Since the dependence of ion mobility on electric field strength is compound specific, this leads to a separation of the different types of ions one from the other, and is referred to as the FAIMS separation or the FAIMS mechanism. In order to transmit an ion of interest through FAIMS, an appropriate direct current compensation voltage (CV) is applied to compensate for the drift of the ion of interest toward the analyzer wall. By varying the CV, different ions are selectably transmitted through the FAIMS device.
In addition to its unique gas-phase ion separation mechanism, an atmospheric pressure ion focusing mechanism also exists when using FAIMS, resulting in high ion transmission. When used with mass spectrometry (MS) and tandem liquid chromatography mass spectrometry (LC-MS), FAIMS provides an extra degree of selectivity without introducing prohibitively large drops in signal intensity, compared with the signal intensity without FAIMS being present. This selectivity is especially important for analyses where several interferences may be present, e.g., in biological samples.
Although there are many instances where FAIMS is used beneficially to improve the selectivity of an analytical method, there also are certain cases where the use of FAIMS is not desirable. Examples include cases where (1) the background of an assay is already very low, and (2) FAIMS is not compatible with the time scale of the analysis. In the first example, FAIMS may be left in operational mode as long as it does not degrade the analysis. However, this may not be desirable when using an established method since additional method development work would need to be done and the new method with FAIMS in operational mode would require validation. The second example is more problematic because FAIMS negatively impacts the analysis. This occurs because typical residence times for ions to pass through a FAIMS device operating at atmospheric pressure in a commercial instrument environment are in the tens of milliseconds range. Consider the case of analyses in which twenty analyte compounds are injected into a column and are subsequently eluted from the column during a finite period of time, typically 2-20 seconds although the actual time may be shorter or longer. Several points for each analyte compound are required to properly define the peak shape, in order to determine the amount of each analyte compound in the sample. When FAIMS is not present, the MS can be set to detect each analyte compound for 10 ms in a looped detection sequence such that a sampling of the amount of all twenty analyte compounds is obtained every 200 ms. For a chromatographic peak width of 2 s, this enables 10 points to be sampled per analyte compound. The situation is quite different when a FAIMS device is present with a 100 ms ion residence time and in which each analyte compound is transmitted at a different CV value. After detecting one analyte compound for 10 ms, the CV must then be changed which is followed by a delay of 100 ms before the next compound passes through the FAIMS to the MS. Accordingly, with FAIMS present it takes 2.2 s to sample just one point for each one of the twenty analyte compounds. Under these conditions, it is possible to miss the elution of an analyte compound entirely, and in any event, too few points are sampled to properly reconstruct the chromatographic peak shapes. As a consequence, for analyses of this type, the use of FAIMS is not desirable.
The need to physically remove the FAIMS hardware each time the use of FAIMS is not desired (typically requiring the same amount of time and effort as changing an ion source) presents a serious impediment to the routine usage of FAIMS. Ideally, the change is achieved either electronically or in the software that operates the system, without physically removing the FAIMS hardware.
In U.S. Pat. No. 6,822,224 filed on Mar. 14, 2001, the entire contents of which is incorporated herein by reference, Guevremont discloses a FAIMS analyzer that is built in the same physical configuration as a quadrupole mass filter analyzer. The quadrupole FAIMS is described for separating isobaric ions, e.g. ions having the same mass-to-charge ratio, which are produced in a collision cell that is disposed immediately in front of the quadrupole FAIMS. When isobaric ions are not produced in the collision cell, and thus ion separation using FAIMS is not required, the quadrupole FAIMS is operated in rf-only mode and its separation function is effectively removed from the system.
In United States Publication 2006/0038121 filed on Sep. 23, 2003, the entire contents of which is incorporated herein by reference, Guevremont discloses a combined rf-only and FAIMS quadrupole cell. In one embodiment the combined rf-only and FAIMS quadrupole cell is disposed directly behind the entrance orifice of the mass spectrometer. The cell is operated alternately in FAIMS separation mode and rf-only mode so as to effect a FAIMS separation and then subsequently focus the remaining ions toward the longitudinal axis for introduction into a detector.
Certainly, a FAIMS analyzer that is based on the same physical configuration as a quadrupole mass filter analyzer does support removal of the FAIMS separation component of an ion analysis path, without physically removing any components of the system. That being said, the ion transmission efficiency that is achievable with the FAIMS “turned off” is lower than the expected ion transmission efficiency when the FAIMS hardware is not present. This is due, at least in part, to a noding effect resulting from the periodic nature of ion motion through a quadrupole and the efficiency of transferring ions through the quadrupole exit lens. In addition to reduced ion transmission efficiency, a FAIMS analyzer based on the same physical configuration as a quadrupole mass analyzer requires a set of precision-machined rods that are rigidly supported in a precise, spaced relationship. Accordingly, the manufacture of this type of FAIMS device is highly specialized and is relatively expensive.
There exists a need for a FAIMS apparatus that overcomes at least some of the above-mentioned limitations.